1. Field of the Invention
This invention relates generally to epicyclic gear assemblies and more particularly to a planetary gear train providing a differential planetary drive which is free of backlash.
2. Description of the Prior Art
Mechanical systems whereby rotary motion is coupled from an input member to an output member are well known and include such devices as harmonic drives and planetary gear systems.
Harmonic drives provide a means for providing high torque amplification in a compact package. Typically, such drives are generally comprised of two types, namely a cup-type drive and a pancake-type drive and consist of a wave generator, a flex spline and a circular spline. When the wave generator is rotated in the clockwise direction, for example, it presses against the flex spline in two diametrically opposite points and distorts the flex spline in the process. These distortions force the gears on the outside of the flex spline to push on their opposite number gears on the inside of the circular spline. This in turn causes the two splines to rotate with respect to one another. The drive mechanism is essentially frictional. With the motion from the inside out, the engagement angles are very steep and efficiency is relatively good, particularly for a cup-type configuration. Also, the torque multiplication is relatively high; however, the flex spline is prone to overstressing and failure via ratcheting. Since the system is frictional in nature, lubrication and efficiency become a problem, particularly in the pancake-type drive.
Planetary gear systems do not provide high torque amplification and are not anti-backlash devices; however, they are compact, strong and have no non-linearities between their input and output motions. Planetary gear systems typically include a sun gear, at least three planetary gears, and an outer ring gear, with a carrier being provided for maintaining a mutual separation between the planetary gears. The torque output can be provided by any of the members, including the carrier, depending upon its design.
Because backlash is known to present a problem, anti-backlash gears are used in certain applications and are typically configured as two identical gears joined together by a torsion spring. These gears counter rotate relative to one another until each seats on the opposite face of their respective gear teeth into which they are fitted. For very light loads, this acts very well. For larger loads, however, the spring is easily overpowered and a situation arises where one of the gears takes the full force of the load first so that it is not truly an anti-backlash system.
Also known in the art is a carrier-less planetary gear transmission. Such a system includes true anti-backlash gears wherein each gear is comprised of two gears which are coupled together by a rifled spline interface. These gears are pushed away from one another by means of a spring, and in the process of moving away from each other, spin with respect to each other on the rifled spline interface until they contact the opposite sides of the gears they come into contact with in the anti-backlash mode. The rifled spline interface, being a locking angle, the anti-backlash gears do not slide back down the rifled spline when they come under load. Hence the system remains a true anti-backlash system.
Further, the prior art also includes a differential planetary transmission system which does not require a carrier. Such a system splits the sun drive gear into two sections, with the lower section being used to counteract planet gear tilt.
Although such systems operate as intended, they all nevertheless have inherent limitations.